Development of Ultra-low Permeable Reservoir in Ansai Oil Field

Abstract

Abstract Generally, the reservoirs with permeability lower than 50×10–3 m2 is called low permeable oil field. Ansai oil field in which the average permeability is about l.29× 10–3 m2 is classified as ultra-low permeable oil field. Despite many low permeable reservoirs were discovered in early days, they haven't been effectively produced due to the complicated production technology and low profit. However, with the development of petroleum industry, the technologies of developing low permeable reservoirs have been improved, particularly, the progress of fracturing and water injection, the production of low permeable reservoirs has been rapidly developed. This paper aims to overview the reservoir, fracturing and water injection, describe the key methods used in production of Ansai ultra-low permeable reservoirs, including the results of water injection. Introduction Ansai oil field is located in the west of China. It was found in 1983.The oil-bearing series are placed in Yanchang group of Triassic system, including Chang 6, Chang 4+5, Chang 3 and Chang 2 layers. This is the largest integral oil field in Shan Gan Ning Basin up to now. The major oil reservoir named Chang 6 is at the depth of 1000 to 1300 m with the air permeability of 1.29×10–3km2(k=0.49×10–3 m2). So it is a typical ultra-low permeable, low pressure and production reservoir. There is no initial output when drilling in the oil reservoir with conventional mud. The formation productivity test shows that the production is below 0.3 to 0.4 t/d when drilling in oil reservoirs with unbalanced technique using oil-based mud or foam mud. After removing damage by conventional fracturing, the initial production is only 2 to 3 t/d and decrease to below 1.5 tons per day after half a year. Oil well is lack of fluid supply and intermittent pumping. The gas is escaped from the formation, wax and scale are seriously deposited in boreholes. So the pump efficiency is low. The surface environment is very complicated with inconvenient transportation. The production is difficult to manage. Because it is a marginal oil field, at the beginning of development, opinions vary to whether the production is efficient to obtain profit. In order to produce Ansai ultra-low permeable reservoirs efficiently, well group, pilot and industrialized test areas were opened to the study of reservoirs, oil reservoir production, completion, fracturing and water injection, so as to decrease drilling cost and greatly improve the effect of fracturing and water injection. The mechanical oil recovery, surface gathering and transportation and cluster drilling are matched. Ansai oil field has been put into industrialized development since 1990. At present, the average output of single well increases to above 4 tId from 1 to 2 t/d in water injected area. The plateau period has been up to 3 years. P. 227